The invention relates to an optically readable information disc which comprises a substrate plate which has on at least one side a layer of a synthetic resin in which a reflecting optical structure has been provided which comprises an information track of information areas situated alternately at a higher level and at a lower level, which areas are covered with a metal reflection layer.
Such a disc is disclosed in Netherlands Patent Application No. 76 11 395 (PHN 8576) filed in the name of the Applicants.
The known disc is constructed from a preferably transparent substrate plate manufactured from a synthetic resin for example in particular polymethylmethacrylate, which plate is provided on at least one side with a layer of lacquer cured by means of radiation, for example, ultraviolet light, in which the information track is present and which is covered with a thin reflection layer of aluminum. A suitable reflection layer is a vapour-deposited aluminum layer having a thickness of 30 nm. The information areas in which audio or video information may be stored, have small dimensions. The difference in height between the areas is approximately 0.1-0.2 .mu.m. The longitudinal dimensions of the areas which vary in accordance with the stored information are roughly 0.3-3 .mu.m. The stored information is read in reflection by means of laser light on the basis of phase differences. The laser light is preferably focused on the optical structure via the transparent substrate.
Netherlands Patent Application No. 78 03 069 (PHN 9076) also filed in the name of the Applicants discloses optically readable multi-layer information discs in which partially permeable reflection layers of, for example, Ag, Ni or Al may be used.
From the investigation which led to the present invention it has been found that the reflection layers of Ag or Al used so far show disadvantages. It has been found, for example, that the stability of the thin layers of Ag and Al is not wholly acceptable. In the applicants opinion the internal stress of these metal layers plays a role, and also the too low resistance against chemical attack of these metal layers.
The investigation into the quality of the optically readable discs and in particular into the quality and durability of the reflection layers has been carried out inter alia, by means of a cyclic moisture test, sometimes termed a tropical test. According to this test the discs are stored for a long test period of at least a few weeks in periodically varying climatological conditions. For this purpose the discs were placed in a climate box and were subjected to 24 hours temperature and moisture cycles in which the discs were kept at a temperature of 45.degree. C. for 8 hours at a relative humidity of 70-90% and then at a temperature of 25.degree. C. for a period of 16 hours at a relative humidity of 100%. It has been found that the quality of a reflection layer of Al is very poor after a test period of 3-5 weeks. The reflection layer showed holes having a diameter of 1 .mu.m or larger. It will be obvious that such holes have a very detrimental influence upon the quality of the, very finely structured stored information. After the above-mentioned test period, the quality of an Ag layer proved to be slightly better as compared with Al, but in itself was still clearly insufficient. It has furthermore been found that an Ag layer adheres insufficiently to the above-mentioned light-cured lacquer layer, in particular a cured layer of an acrylate or methacrylate synthetic resin.
The above-described problems can now be explained at least partly from the fact that the reflection layer provided has a very small thickness of approximately 70 nm. The properties of the metal, in this case Ag or Al, in such a thin layer clearly differ from the properties of the same metals in bulk. For example, the oxidation sensitivity, the sensitivity to attack by contaminations in the atmosphere and the sensitivity to attack by those components of the underlying cured lacquer layer which have not been polymerized during the exposure to light (however small the quantity of such monomers present in the cured lacquer may be) are obviously increased. Furthermore, the expansion of the synthetic resin present below the metal layer as a result of temperature variations (thermal expansion) or as a result of moisture absorption is higher by a factor of 2 to 5 than that of the metal layer. From this the conclusion might be drawn that a metal layer of the above-mentioned small thickness cannot form a really stable reflection layer on a synthetic resin substrate in which stability must be related to the very finely detailed optical structure.